Multifocal lenses with refractive and diffractive powers are known from U.S. Pat. No. 6,536,899. These lenses have annular or circular-annular zones, wherein each of these annular zones is divided into one main sub-zone and one phase sub-zone. The system of the main sub-zones represents a diffractive lens having two principal powers. The refractive powers in the phase sub-zones are selected such that the averaged refractive power of the entire zone or of the entire lens corresponds to one of the two principal diffractive powers. In contrast to conventional diffractive lenses, the lens according U.S. Pat. No. 6,536,899 does not have any topographic or optical steps on the lens surface.
In U.S. Pat. No. 6,536,899, trifocal lenses are also described, in which the averaged refractive power is equal to the average of the three principal powers, in which the greatest principal power is given by the diffractive power of +1st order, and in which the smallest principal power is given by the diffractive power of −1st order.
Trifocal lenses of the described type have longitudinal chromatic aberrations both in the smallest and in the greatest of the three principal powers. If such lenses are to be employed as ophthalmic lenses (e.g. contact lenses, intraocular lenses), then, in particular the longitudinal chromatic aberration in the smallest of the principal powers is disadvantageous. Namely, this power is then used for imaging far objects, and a longitudinal chromatic aberration associated with the −1st diffraction order is particularly disturbing in such a use. Multifocal lenses with more than two principal powers are specifically desired in the area of ophthalmology since they allow sharp vision in the far distance, in the intermediate distance and in the reading distance. Besides the trifocal lenses according to U.S. Pat. No. 6,536,899, other trifocal lenses are known. In U.S. Pat. No. 5,344,447 trifocal diffractive lenses are described, further also in U.S. Pat. No. 5,760,871. A further trifocal lens is described in United States patent application publication 2008/0030677 A1.
The trifocal lens according to U.S. Pat. No. 5,344,447 has a minimum principal diffractive power equal to the −1st diffractive power with longitudinal chromatic aberration. Furthermore, this lens has the topographic or optical steps on at least one of the lens surfaces, usual in diffractive lenses.
The trifocal lens according to U.S. Pat. No. 5,760,871 also has a minimum principal diffractive power, which corresponds to the −1st diffraction order with longitudinal chromatic aberration.
The trifocal lens according to United States patent application publication 2008/0030677 A1 has a minimum principal diffractive power, which corresponds to the zeroth diffraction order, and a maximum power, which corresponds to the first diffraction order of the diffractive lens. According to this prior art, light is directed to a location between the two foci of these powers by a certain design of adjacent diffraction steps. As all of the conventional diffractive lenses, this lens has topographic steps or optical steps on one of the two lens surfaces.
Topographic steps on a lens surface are disadvantageous for several reasons: usually, such steps are difficult or not to be produced with the required precision. Furthermore, such steps are detrimental to the wearing comfort in ophthalmic lenses such as contact lenses.
A diffraction lens or diffractive lens generally consists of a number of circular-annular lens zones of each identical area; usually, such zones are called Fresnel zones. Between adjacent zones, usually, steps with the path length differences t associated therewith are provided, wherein these path length differences are usually smaller than a design wavelength λ. The area or size of the zones determines the separations between the diffractive powers of the lens, wherein these separations increase with decreasing area of the zones. The optical path length difference t determines the relative maximum intensities in the individual diffractive powers, for example, at t=λ/2, there are two principal diffractive powers, which correspond to the zeroth and the first diffraction order, and both have a maximum intensity of (2/π)2=40.5%, wherein 100% is the maximum intensity of a lens limited in diffraction with identical Fresnel zones, but no steps between the zones. The latter lens is a “normal” refractive lens. For path length differences, which are absolutely smaller than a half design wavelength, the power of the zeroth order dominates, in the case of abs(t)>λ/2, the power of the first diffraction order has the greatest relative intensity.
It is extremely important to note that a refractive power is associated with each individual Fresnel zone of a diffractive lens; this refractive power can be calculated by refraction of an incident light beam with application of the Snell's refraction law. The individual Fresnel zone can have a uniform power, but it also can have a surface configuration to the effect that the refractive power varies along the zone surface; then, the refractive power of such a zone is an average power.
In conventional multifocal diffractive lenses with optical steps between contiguous zones, none of the diffractive powers is identical to the refractive powers of the zones. In particular, this also applies to the zeroth diffractive power of a diffractive lens.
There are two fundamental formations of diffractive lenses. In the first formation, the path length difference t between the first and the second zone is equal to that between the second and the third zone, and so on. Embodiments of such diffractive lenses usually have a saw tooth profile on one of the two surfaces of a lens manufactured with a given refractive index. In the second fundamental formation of diffractive lenses according to the prior art, the optical path length differences are +t between the first and the second zone, −t between the second and the third zone, +t between the third and the fourth zone, and so on. The disadvantages of such known diffractive lenses are explained in U.S. Pat. No. 6,536,899.
In U.S. Pat. No. 6,536,899, lenses are mentioned according to the invention there, which are formed without topographic and optical steps on the lens surface. In this context, a trifocal lens is also mentioned, in which the individual zones have different averaged powers, and moreover disadvantageously longitudinal chromatic aberrations occur both in the smallest and in the greatest of the three principal powers.